The technology of the dual-energy electron linear accelerator is widely applied in imaging inspection systems such as containers/vehicles inspection systems etc. Substances can be distinguished using the difference between the attenuation characteristics of both high energy X-ray and low energy X-ray passing through materials with different atomic coefficients. A dual-energy X-ray inspection system requires emitting a beam at two energy levels. As a repetition frequency is typically on the order of 100 Hz, the energy is required to be switched on the order of milliseconds. The electron linear accelerator is typically comprised of sub-systems such as a microwave power source, a microwave transmission system, an electron gun, an accelerating tube etc. The existing methods for adjusting the energy of the accelerator primarily comprise: changing the energy by 1) changing a magnitude of microwave power input into the accelerator; 2) changing a magnitude of a beam load of the accelerator; and 3) changing a distribution of a part of electromagnetic field in the accelerator by designing an accelerating structure in a particular manner.
In the method 1), the manner of directly changing the magnitude of the energy of the power source input into the accelerator is easy and feasible. However, when power output from the microwave power source is switched rapidly, a condition that the frequency changes and the output power is unstable may occur for the microwave power source.
In the method 2), the beam load of the accelerator is regulated by changing a current emitted by the electron gun, and the energy of the electron beam is reduced by absorbing more microwave power by a stronger beam. However, as a dosage rate is directly related to the magnitude of the beam, the regulation of the parameter is less flexible; and at the same time, the demands on the electron gun also increase.
In the method 3), the accelerating structure is usually very complex, and it generally needs to regulate the hardware structure of the accelerating tube so as to regulate the field distribution of the accelerator, which has a slow time response.
Further improvements may be made to the method for adjusting an energy based on the method 1). The improved method changes the microwave power input into the accelerating tube while the microwave power source operating in the same state, so as to ensure that the frequencies of the microwave source at the two energy levels are consistent and the output power is stable. This solution needs to add a microwave transmission system which can switch the attenuation or gain rapidly between the accelerating tube and the microwave source.
US patent US20100039051 discloses a method based on a magic-T element. An arm in the magic-T is connected to a phase shifter. The power input into the accelerating tube is regulated by changing a reflection phase of the arm rapidly to change a division ratio between power input into two output ports during power synthesis with another arm. However, the method operates in a total-reflective pure standing wave state, the power capacity is limited, and the demands on the circulator are very high. In addition, only an output less than the power of the power source can be achieved with this method.